The engine MM states that wear felt with the radius scribe is reason for rejecting and replacing the torquemeter gear, and Rolls-Royce indicated that the radius scribe will typically detect wear of 0.001 to 0.002inch. Therefore, it could be concluded that the in-service wear limit would be 0.001 to 0.002inch. Rolls-Royce has also indicated that, because the gearbox is maintained on-condition, the gears could far exceed the 3500-hourMPI requirement. The occurrence torquemeter gear had 0.004inch wear, at least twice the in-service wear limit, and had failed at 4576hours, 1076hours beyond the MPI inspection requirement. The torquemeter gear failed as a result of an undetected crack that progressed in fatigue. MPI is a method of non-destructive testing used to visually identify cracks originating at the surface in steel components. If an MPI had been carried out on the torquemeter gear before its failure then it is likely that the crack would have been detected. However, the requirement to perform the MPI is based specifically on the time-in-service of the torquemeter gear, and then only if the gearbox is already opened. There is no requirement to track the time-in-service for the torquemeter gear, or for any of the other gears in the gearbox. Therefore, if a serviceable gear is installed during gearbox maintenance, it is possible that the gear's time-in-service may not be determined. Rolls-Royce has reported that, for some operators, the MTBR of the torquemeter gear is 650hours, and SDR data indicate that torquemeter gears have been rejected with as few as 154hours. The longer a gear remains in service without being inspected, the greater the risk of failure, such as the occurrence torquemeter gear. In October 2004, Rolls-Royce introduced a redesigned torquemeter gear to eliminate premature wear. In August2007, Rolls-Royce introduced another redesigned torquemeter gear to improve the reliability. Because the gears identified in CEB 72-6061 are not considered as mandatory replacements, operators can continue to use and install serviceable part number6893673 torquemeter gears, although it is possible that the gear's time-in-service may not be determined.Analysis The engine MM states that wear felt with the radius scribe is reason for rejecting and replacing the torquemeter gear, and Rolls-Royce indicated that the radius scribe will typically detect wear of 0.001 to 0.002inch. Therefore, it could be concluded that the in-service wear limit would be 0.001 to 0.002inch. Rolls-Royce has also indicated that, because the gearbox is maintained on-condition, the gears could far exceed the 3500-hourMPI requirement. The occurrence torquemeter gear had 0.004inch wear, at least twice the in-service wear limit, and had failed at 4576hours, 1076hours beyond the MPI inspection requirement. The torquemeter gear failed as a result of an undetected crack that progressed in fatigue. MPI is a method of non-destructive testing used to visually identify cracks originating at the surface in steel components. If an MPI had been carried out on the torquemeter gear before its failure then it is likely that the crack would have been detected. However, the requirement to perform the MPI is based specifically on the time-in-service of the torquemeter gear, and then only if the gearbox is already opened. There is no requirement to track the time-in-service for the torquemeter gear, or for any of the other gears in the gearbox. Therefore, if a serviceable gear is installed during gearbox maintenance, it is possible that the gear's time-in-service may not be determined. Rolls-Royce has reported that, for some operators, the MTBR of the torquemeter gear is 650hours, and SDR data indicate that torquemeter gears have been rejected with as few as 154hours. The longer a gear remains in service without being inspected, the greater the risk of failure, such as the occurrence torquemeter gear. In October 2004, Rolls-Royce introduced a redesigned torquemeter gear to eliminate premature wear. In August2007, Rolls-Royce introduced another redesigned torquemeter gear to improve the reliability. Because the gears identified in CEB 72-6061 are not considered as mandatory replacements, operators can continue to use and install serviceable part number6893673 torquemeter gears, although it is possible that the gear's time-in-service may not be determined. The helical torquemeter gear failed as a result of an undetected crack that progressed in fatigue. The failure of the torquemeter gear resulted in the loss of engine power to the helicopter's transmission.Finding as to Causes and Contributing Factors The helical torquemeter gear failed as a result of an undetected crack that progressed in fatigue. The failure of the torquemeter gear resulted in the loss of engine power to the helicopter's transmission. In-service wear may cause torquemeter gear part number6893673 to wear prematurely. Because it is not mandatory to replace torquemeter gear part number6893673 with the newly released torquemeter gear, it is possible that torquemeter gear part number6893673 will experience premature wear and failure. The Rolls-Royce 250-C47B Operation and Maintenance Manual inspection requirements allow the torquemeter gear and other gears installed in the gearbox to potentially exceed 3500hours in service before a magnetic particle inspection (MPI) is carried out. The current visual and radius scribe inspections may be inadequate to detect cracks in the gear teeth. The Rolls-Royce Operations and Maintenance Manual identifies the requirement for an MPI on the torquemeter gear and other gears in the gearbox based on their time-in-service. However, there is no requirement to track the time-in-service for any of these parts.Findings as to Risk In-service wear may cause torquemeter gear part number6893673 to wear prematurely. Because it is not mandatory to replace torquemeter gear part number6893673 with the newly released torquemeter gear, it is possible that torquemeter gear part number6893673 will experience premature wear and failure. The Rolls-Royce 250-C47B Operation and Maintenance Manual inspection requirements allow the torquemeter gear and other gears installed in the gearbox to potentially exceed 3500hours in service before a magnetic particle inspection (MPI) is carried out. The current visual and radius scribe inspections may be inadequate to detect cracks in the gear teeth. The Rolls-Royce Operations and Maintenance Manual identifies the requirement for an MPI on the torquemeter gear and other gears in the gearbox based on their time-in-service. However, there is no requirement to track the time-in-service for any of these parts. On 17 August 2007, Rolls-Royce issued Commercial Engine Bulletin (CEB) 72-6061, which advised customers of the 250-C30 and 250-C47series engines that the power gears (pinion gear, torquemeter gear, and power take-off gear) had been redesigned to improve the reliability of the new gears. The CEB states that compliance is a customer option. On 26 March 2008, Rolls-Royce advised that it is developing a visual inspection to be placed into the 2000-hour inspection section in the Operation and Maintenance Manual. Implementation is targeted for the third quarter of 2008. On 23 May 2008, Transport Canada advised that a Service Difficulty Alert regarding this issue has been approved and will be published by the end of July2008.Safety Action Taken On 17 August 2007, Rolls-Royce issued Commercial Engine Bulletin (CEB) 72-6061, which advised customers of the 250-C30 and 250-C47series engines that the power gears (pinion gear, torquemeter gear, and power take-off gear) had been redesigned to improve the reliability of the new gears. The CEB states that compliance is a customer option. On 26 March 2008, Rolls-Royce advised that it is developing a visual inspection to be placed into the 2000-hour inspection section in the Operation and Maintenance Manual. Implementation is targeted for the third quarter of 2008. On 23 May 2008, Transport Canada advised that a Service Difficulty Alert regarding this issue has been approved and will be published by the end of July2008.